1. Field of the Invention
This invention relates generally to an improvement in the field of culturable air sampling for biological materials, and more particularly to a combination air sampling cassette and nutrient media dish for the selective collection of viable microorganisms from ambient air for culturing and analysis.
2. Discussion of Related Art
The growing awareness of the potential adverse health effects of microorganisms has given rise to an increased need to detect and quantify airborne microorganisms during evaluation of indoor air quality. As is well-known, many respirable viable particulates are entrained in the air we routinely breathe. The prior art includes a number of ambient air samplers adapted for use in collecting such viable microorganisms. In operation, these devices generally enable the collection of viable microorganisms onto a culture media or nutrient to stimulate incubation and foster colony growth. Subsequent laboratory analysis will identify and enumerate the colonies.
A well-respected device in wide use in the field of viable sampling is the Andersen viable (microbial) sampler, disclosed in U.S. Pat. No. 3,001,914. The patent teaches a mechanism which serves to count and classify microorganisms in air. The device comprises a series of stages, each stage including a perforated member positioned over a layer of nutrient media. Airborne particles are impacted onto the nutrient when air is drawn through the device. On incubation the viable particles in the nutrient become visible as colonies. The device is available as a six-stage or two-stage system when particle sizing is required, or single-stage when particle sizing is not required.
The Andersen sampler has two fundamental disadvantages: it is expensive and it is inconvenient to use. Several commercially available devices have been developed to overcome the limitations of the Andersen sampler, including, for example, the RCS(trademark) centrifugal air sampler, the PBI surface air system sampler (SAS), and the Mattson-Garvin Slit-to-Agar air sampler. (RCS is a registered trademark of Biotest AG Corporation, Federal Republic of Germany.) While these devices arguably provide greater ease of use, they are still expensive and, more significantly, do not exhibit the collection efficiency of the Andersen sampler [Reference:xe2x80x9cEvaluation of Eight Bioaerosol Samplers Challenged with Aerosols of Free Bacteriaxe2x80x9d, American Industrial Hygiene Association Journal (53), October 1992.]
Other devices similar to the Andersen design have been developed, including U.S. Pat. No. 3,922,905 and 4,038,057, both to Roth. The ""057 patent teaches a sampling device for removing particulate matter from gaseous media by jet impaction. The sampler includes a base, and impaction stage, and a connector diffuser connected to the impaction stage. The impaction stage has a plate with apertures for generating a prescribed velocity of the gas as it passes through the apertures. A nutrient medium plate is positioned under the apertured plate and the gas passing through the apertures impacts the nutrient medium such that airborne particulates are captured. The Roth devices were intended to have lower equipment cost and be simpler to use, but they have not displaced the Andersen sampler as the device of choice, most probably because they are not, in fact, appreciably easier to use than the Andersen sampler.
Accordingly, there remains a need for a viable sampler that has demonstrable equivalence to the Andersen sampler in terms of performance but which has the advantages of ease of use and lower cost.
Objects and Advantages
Accordingly, the primary objects and advantages of the combination air sampling cassette and nutrient media dish of the present invention include:
1. to provide a viable sampler that has lower equipment cost;
2. to provide a viable sampler that is faster and easier to use; and
3. to provide a viable sampler that is smaller and lighter than currently known devices.
Further objects and advantages of the invention will become apparent from a consideration of the drawings and ensuing description.
The combination air sampling cassette and nutrient media dish of the present invention generally comprises a base and media dish assembly that defines an upwardly opening recess therein, an air outlet therefrom, and an enclosure retaining culture media for the collection of particles. In a first preferred embodiment, an orifice plate comprising a plate with a plurality of small holes fits onto the integrated media dish in a sealing arrangement. A pump is connected to the air outlet in the base to pull air through the orifice plate, over the culture media, and out through the air outlet. As the air passes through the holes in the orifice plate it is accelerated and results in the selected impaction of particles in the culture media. A cover fits over the assembly to protect the culture media prior to and after sampling.